1. Field of the Invention
The present invention relates to a machine tool rotational indexing device where a rotary section loaded with a workpiece or a tool is rotationally and indexably mounted in a specified rotational angle position and it is clamped in the indexed position on a fixed base.
2. Discussion of the Related Art
A horizontal machining center is sometimes provided with a rotational indexing device that has a processing table loaded with a workpiece. The processing table is rotationally indexed and the workpiece can be clamped in a specified rotational angle position which allows processing to be performed in this state.
In a rotational indexing device of this type, a 3-piece coupling type is provided. This includes a rotary-side coupling fixed to the processing table and a stationary-side coupling fixed to a fixed base. The rotary-side coupling and the stationary-side coupling are coaxially arranged. A joint coupling is arranged to move between a clamped position and an unclamped position. In the clamped position, the joint coupling is in engagement with both the rotary-side coupling and the stationary-side coupling and in the unclamped position, the joint coupling is spaced therefrom so that the meshing engagement is released.
Conventionally, in the above-mentioned rotational indexing device, it is sometimes the case where the rotary-side coupling is arranged outside the stationary-side coupling and rollers are interposed between both the couplings. Moreover, in an effort to improve the support rigidity of the rotary-side coupling, sometimes a structure for supporting the rotary-side coupling on a fixed base in both the clamped and unclamped positions by pressurizing and putting the coupling into sliding contact with the fixed base is adopted (see for example, Japanese Patent Laid-Open Publication No. HEI 7-256538 and Japanese Patent Laid-Open Publication No. HEI 9-19848).
However, the aforementioned conventional rotational indexing device, which puts the rotary-side coupling in sliding contact with the fixed base, has a concern about the possible occurrence of partial wear when an unbalanced load acts on the rotary-side coupling. Moreover, it is sometimes the case where a slide member or a hollow roller is provided on the sliding surface of the fixed base in order to reduce the sliding resistance. If this arrangement is adopted, there is a problem that the part count increases.
Accordingly, in view of the aforementioned conventional problems, the present inventor (with other coinventors) proposed a machine tool rotational indexing device where a plurality of rolling element rows are arranged between the rotary-side coupling and the stationary-side coupling, and an outer surface of the rotary-side coupling is only supported on the stationary-side coupling via the rolling element rows. This device was filed as Japanese Patent Application No. 2000-359491 and now has been published as Japanese Patent Laid Open Publication No. 2002-160144 (and is not prior art for this application).
According to the rotational indexing device of the aforementioned proposal, the plurality of rolling element rows are arranged between the rotary-side coupling and the stationary-side coupling. Therefore, the clamping force and the load that acts on the rotary section can be supported on the stationary-side coupling via the rolling elements of each row, and the support rigidity provided by the fixed base of the rotary-side coupling can be improved.
The device of the aforementioned proposal adopts a structure for imposing a preload force on the rolling elements of each row in a state where the joint coupling is in the unclamped position. However, it was discovered that the preload force of the rolling elements of any of the rows was reduced when the joint coupling was moved to the clamped position depending on the manner of imposing the preload force or the magnitude of the clamp load by the joint coupling. This sometimes caused a problem that sufficient support rigidity of the rotary-side coupling was not able to be obtained.